Abstract
Water quality impairment by elevated sediment loss is a pervasive problem for global water resources. Sediment management targets identify exceedance or the sediment loss ‘gap’ requiring mitigation. In the UK, palaeo-limnological reconstruction of sediment loss during the 100–150 years pre-dating the post-World War II intensification of agriculture, has identified management targets (0.20−0.35 t ha−1 yr−1) representing ‘modern background sediment delivery to rivers’. To assess exceedance on land for grazing ruminant farming, an integrated approach combined new mechanistic evidence from a heavily-instrumented experimental farm platform and a scaling out framework of modelled commercial grazing ruminant farms in similar environmental settings. Monitoring (2012–2016) on the instrumented farm platform returned sediment loss ranges of 0.11−0.14 t ha−1 yr−1 and 0.21−0.25 t ha−1 yr-1 on permanent pasture, compared with between 0.19−0.23 t ha−1 yr-1 and 0.43−0.50 t ha−1 yr−1and 0.10−0.13 t ha−1 yr−1and 0.25−0.30 t ha−1 yr-1 on pasture with scheduled plough and reseeds. Excess sediment loss existed on all three farm platform treatments but was more extensive on the two treatments with scheduled plough and reseeds. Excessive sediment loss from land used by grazing ruminant farming more strategically across England, was estimated to be up to >0.2 t ha−1 yr−1. Modelled scenarios of alternative farming futures, based on either increased uptake of interventions typically recommended by visual farm audits, or interventions selected using new mechanistic understanding for sediment loss from the instrumented farm platform, returned minimum sediment loss reductions. On the farm platform these were 2.1 % (up to 0.007 t ha−1 yr−1) and 5.1 % (up to 0.018 t ha−1 yr-1). More strategically, these were up to 2.8 % (0.014 t ha−1 yr−1) and 4.1 % (0.023 t ha−1 yr−1). Conventional on-farm measures will therefore not fully mitigate the sediment loss gap, meaning that more severe land cover change is required.
Highlights
Improved pasture and rough grazing accounts for ~67 % of the agricultural land area of the UK (Defra, 2016), supporting production with a net worth of ~£8 billion to the UK economy (Orr et al, 2016)
On the grass/clover reseeded mix treatment, total sediment loss ranged between 1.09–1.32 t, equating to 0.19− 0.23 t ha− 1 yr− 1 and 9.89–11.64 t or 0.43− 0.51 t ha− 1 yr− 1
The work reported illustrates that two shortlists of recom mended on-farm measures selected to capture either visual audits or to address mechanistic understanding for soil loss and sediment delivery on the soils in question, are unable to close the exceedance or sediment loss gap
Summary
Improved pasture and rough grazing accounts for ~67 % of the agricultural land area of the UK (Defra, 2016), supporting production with a net worth of ~£8 billion to the UK economy (Orr et al, 2016). Much of the work in the UK investigating soil erosion on agricultural land and, its on-site and off-site consequences, originally focused on arable production systems (Evans, 1971, 1990), since the late 1990′s, there has been growing recognition of the important role of modern intensive ruminant farming in generating soil erosion and sediment problems (Collins et al, 1997; Evans, 1998; Walling, 2005; Bilotta et al, 2008; Granger et al, 2010; Peukert et al, 2014). The environmental impact of agriculture, including externalities related to soil erosion and sediment loss, is under continued scrutiny (Montgomery, 2007; FAO, 2011; Whitecraft and Huggins, 2013; Montanarella, 2015; Borrelli et al, 2017)
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